The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for fully phase-encoded three-dimensional MRI using multiband radio frequency (“RF”) excitation without using frequency-encoding gradients.
Magnetic resonance imaging (“MRI”) of non-ferrous metallic implants is challenging because of the substantial inhomogeneity induced in the B0 magnetic field of the MRI system. This inhomogeneity leads to severe off-resonance in nearby tissue, causing signal loss and image distortion when using conventional spatial-encoding mechanisms. The image artifacts resulting from the off-resonance can significantly degrade the diagnostic quality of an image, making clinical diagnoses in the presence of metal very challenging.
This extreme off-resonance is problematic for conventional MRI applications for at least two reasons. First, the spectrum of off-resonance induced by metallic objects, such as metallic implants, is often so wide that it is impossible to excite the full spectrum of frequencies with a single RF pulse. Second, frequency-encoding in areas of extreme off-resonance can lead to signal pile-up and signal loss because the spatial encoding assumptions are violated.
Recently, new methods, such as slice-encoding for metal artifact correction (“SEMAC”) and multi-acquisition variable-resonance image combination (“MAVRIC”), have been developed in an attempt to mitigate the off-resonance artifacts surrounding metallic implants. These methods are described, for example, by K. M. Koch, et al., in “Magnetic Resonance Imaging Near Metal Implants,” J Magn Reson Imaging, 2010; 32(4):773-787.
MAVRIC and SEMAC techniques utilize frequency-encoding gradients (e.g., readout gradients) for spatial localization, which limits the ability of these methods for performing MRI near metal. When the local B0 gradients near metal exceed the readout gradient, signal loss and pile-up are inevitable with frequency-encoding. This phenomenon is described by K. M. Koch, et al. in “Imaging Near Metal: The Impact of Extreme Static Local Field Gradients on Frequency Encoding Processes,” Magn Reson Med, 2013; Jul. 10 doi: 10.1002/mrm.24862. [Epub ahead of print].
Fully phase-encoded (“FPE”) imaging techniques eliminate frequency-encoding gradients by encoding k-space one point at a time. Previously, FPE techniques were proposed in an effort to produce distortion-free images in the presence of off-resonance. This effort, however, has not gained traction because of the prohibitively long scan times associated with FPE methods. A recent FPE method that is capable of spectrally-resolved, purely phase-encoded (“SR-FPE”) three-dimensional acquisitions was recently proposed, as described in co-pending U.S. patent application Ser. No. 13/451,773, filed on Apr. 20, 2012, entitled “System and Method for Spectrally-Resolved Three-Dimensional Magnetic Resonance Imaging Without Frequency-Encoding Gradients,” and which is herein incorporated by reference in its entirety.
Feasibility for reducing a four hour SR-FPE scan to twelve minutes using parallel imaging in all three spatial dimensions (as opposed to using parallel imaging in only two directions, as is the current convention) was demonstrated for a single RF offset by N. S. Artz, et al., in “Spectrally Resolved Fully Phase-Encoded Three-Dimensional Fast Spin-Echo Imaging,” Magn Reson Med., 2013; Mar. 11. doi: 10.1002/mrm.24704. [Epub ahead of print]. However, similar to MAVRIC and SEMAC, multiple three-dimensional acquisitions at distinct RF offsets will be required to excite the full spectrum of off-resonance for most metallic implants, requiring an increase in scan time that may be clinically prohibitive.
Thus, there remains a need for a system and method for magnetic resonance imaging that is capable of accelerating data acquisitions that require multiple RF offsets due to severe off-resonances, such as those caused by magnetic field inhomogeneities induced by a metallic object.